In high-speed digital transmission the degradation of the signal is mainly due to the so-called "intersymbol interference" caused by the fact that, owing to non-ideal characteristics of the transmission channel, every pulse containing the digital information is associated with leaders or precursors and with tails or postcursors which overlap and distort adjacent pulses. Such interference, increasing sharply with speeds exceeding twice the Nyquist rate, significantly enhances the probability of incorrect decisions on pulse levels at the receiving end of the transmission path. The resulting distortions of the conveyed intelligence is the greatest obstacle to a fuller utilization of available transmission lines.
In my prior U.S. Pat. No. 4,170,758 and in my copending application Ser. No. 065,468, filed Aug. 10, 1979, I have disclosed an equalizer of the nonlinear type designed to compensate for both precursor and postcursor effects. Reference may also be made to the art of record in that prior patent, including an article by Tonau Osatake and Hidehiko Tanaka entitled "Error-Rate Improvement Through Digital Decision in Pulse Transmissions," Electronics and Communications in Japan, Vol. 49, No. 10, published October 1966 (pages 28-35).
The term "nonlinear equalization" refers to a circuit arrangement in which a distorted incoming signal is fed to a decision stage which converts it, on the basis of predetermined threshold levels, into a quantized pulse supplied to a filter (e.g. one of the well-known transversal type) deriving therefrom an estimated feedback or feed-forward pulse for postcursor or precursor compensation of a succeeding or a preceding signal, respectively. Linear filtering, on the other hand, dispenses with the decision stage so that the corrective pulses are of analog character. An advantage of the nonlinear technique is the reduced error rate in the last quantizing step to which the corrected signal is subjected in a final decision unit. Any error that does occur in an earlier threshold stage, however, is likely to proliferate in the processing of subsequent signals.